Document: GLE-3-34-11

Nitrogen deposition and NPP in temperate forests: The role of leaf physiology, foliage turnover and canopy structure.

BERNTSON, G.* ¹, G.BAUER ² and R.MINOCHA ³

University of Nwq Hampshire, Durham, NH, 03824, USA ¹
Harvard University, Cambridge, MA, 02138, USA ²
USDA Forest Service, NERS, Durham, NH 03824, USA ³

Abstract:
There is growing evidence to suggest that the impact of N deposition on temperate forest productivity, even in the absence of N saturation, may not be as great as several studies suggest. Further, most studies of the impact of N deposition on forest processes have ignored changes which take place in the canopy. To date, there have been no empirically-based, ecosystem-scale studies of how N deposition alters canopy processes and thus the potential for C gain in forests. We initiated a study of how N deposition affects the physiology, dynamics, and structure of temperate forest canopies. We are examining how the partitioning of N within tree foliage and how it is affected by N deposition, and how these changes in N allocation relate to changes in photosynthetic capacity. In addition we are examining the effects of N deposition on foliage retention time and canopy structure in order to begin scaling changes in leaf-level processes to the entire canopy. Our study is based at the Harvard Forest Chronic N Experiment, where a long-term (12-yr+) of N deposition (+5 and +15 g N m^-2 y^--1) on both coniferous and deciduous forests is underway. Our preliminary measurements (within a Pinus resinosa stand) demonstrate that foliar N content has significantly increased, and that this increase is accompanied by a de-coupling of the Photosynthesis-N relationship. Preliminary evidence suggests that the de-coupling is due either to increased production of non-photosynthetic proteins (e.g., Calvin cycle enzymes) or an excess production of Rubisco. The net effect of this de-coupling is that the large increases in foliar N we have observed are not accompanied with increased photosynthetic capacity. Further, we have found that needle life-span has been significantly reduced, and, even with apparent increases in foliar production (at the expense of wood production), standing leaf area has decreased. Taken together, these data suggest that potential C gain by the Pinus resinosa canopy has been significantly reduced by long-term N deposition.

Keywords: Temperate Forest, Canopy, Photosynthesis, Leaf Area Index, Leaf Turnover, Rubisco, Nitrogen

This abstract is being presented at: 2:45 PM in session:
Oral Session #28: Effects of N Deposition in Ecosystems.